We know Leonardo da Vinci the best, I think, through his sketchbooks. He was a voracious learner and thinker, interested in everything. His sketchbooks are not merely studies for paintings, but a record of ideas, the appearance of people and things around him, inventions, anatomical studies.
The marks he makes are all dauntingly lovely (Leonardo did not seem to make any ugly marks as far as I have been able to tell). But the variety of marks is reassuringly workmanlike. In this sketchbook page, the marks describing the horse and rider feel different than the ones honing the man’s head. You can almost feel how fast the horse is moving—soon he will be off the page. These marks are an exploration of that, I think. But the man’s head is carefully worked out, and gridded for easy transferral to a work that is outside the pages of the sketchbook.